There is difference of opinion as to the cause of the steady decline in temperature, which marked the oncoming of that period of geological history, know as the Quarternary epoch, the Pleistocene or Glacial Age. A gradual change in the inclination of the earth’s axis has been suggested by some students, and apparently disproved by others.

Locally, the lengthening of the mountain line, which separated us from the western coast, the diversion of the Japan Current from our borders, together with a constant increase in the elevation of our mountain ranges helped, if indeed they did not cause the long periods of excessive cold.

While the causes are obscure, the methods and processes of nature during the fiercely cold centuries, and their results to the country of Idaho, are as an open book.

After the period of volcanic action – just how long after is only conjecture - another great geologic change came to the region now comprising Idaho. During the latter part of the Tertiary period there was a general lowering of the temperature in the north temperate zone until the climate along the forty-fifth parallel of latitude was not unlike that in the vicinity of the Arctic Circle at the present and in consequence of the extreme cold and heavy snowfall immense glaciers were formed in the valleys throughout all the central portion of North America. Then followed the period known to geologists as the Pleistocene or “Ice Age”, the last great important geologic change, extending far into the Quarternary era. As the temperature again rose, the glaciers began slowly moving toward a lower altitude. In their progress they carried along with them soils, great boulders, etc., and deposited these far from the place where they had been first placed by the hand of Nature. The ridges formed in many places by this debris are called moraines. The ridge left along the edge of the glacier is called a “lateral moraine”; that formed where two glaciers came together, a “medial moraine”; and that where the last of the ice was dissolved by the rising temperature, a “terminal moraine’. Geologists are able to determine easily by the character of the moraine the class to which it belongs, and thus form a definite idea of the magnitude of the glacier and the direction in which it moved.

There are abundant evidences of glacial activity in the region of the Lost River and Sawtooth Mountains. In the upper portions of the valley glacial boulders and lateral as well as terminal moraines can be found. East of the Sawtooth Range enormous deposit of glacial drift, extending along the foot of the mountains for fifty to sixty miles in a belt form five to eight miles in width, cut by many streams. Glacial action is equally well marked in numerous localities in the central and northern portion of the state.

Some geologists think that the glacial invasion of Central North America lasted for 500,000 years, and that the last of the glacial ice in the United States disappeared at least 25,000 years ago. At the close of the Pleistocene period the affected part of the earth’s surface was barren of vegetation and animal life. The water from the melting ice settled in the depressions and formed glacial lakes. In Idaho there are nearly six hundred square miles of lakes, many of which are of glacial origin. Prairie Basin, between the Salmon River Mountains and the middle fork of the Salmon River, is the site of an ancient glacial lake.


Sources:
"Idaho: The Place and Its People" by Bryon Defenbach
"History of Idaho" by James H. Hawley